Where can I buy Windows product key from in the Uk installation will be via USB
Select your country from the drop down list,
BROWSE BY Individual Parts > Operating Systems.
Windows 10 Home: https://uk.pcpartpicker.com/product/Mk648d/microsoft-os-kw900265
Windows 10 Pro: https://uk.pcpartpicker.com/product/CYH48d/microsoft-os-fqc09131
Let me know if you want Windows 8.1 or Windows 7 instead.